This invention relates to the calcination of carbonaceous materials, particularly petroleum coke such as intended to provide carbon for making electrodes or the like. Carbonaceous materials contemplated by the invention, including petroleum coke, can also be defined as those having a volatile content up to about 15% and calcinable to a density of at least about 1.6 g/cc, for example, 2 g/cc or higher. Thus anthracite coal can be considered an example of such material, but ordinary bituminous coal is not.
Calcining operations of this sort are commonly performed in a rotary kiln into which the green petroleum coke in suitable particulate form is fed at or near one end, for delivery of treated product at the other end. In the kiln, the coke is calcined at high temperature, to drive off the volatiles and moisture and re-orientate the crystalline structure of the coke to a predetermined, desired degree. The calcined product is useful for carbon elements and structures, notably for various situations of electrical function, such as in high temperature electrochemical operations, and most particularly for anodes and lining compositions in aluminum reduction cells.
The calcining process requires adequate heating for a desirably high production rate of calcined coke, while at the same time the heating is very preferably achieved inside the kiln without substantial combustion of the carbon itself. As will be understood, the green, granular coke entering the feed end of the tubular kiln flows down the kiln at a rate depending mainly on the kiln slope, for example falling 0.5 inch per foot of run from feed end to discharge end, on diameter, for example from 6 to 15 feet, and on the kiln speed of rotation, for example in the range of 0.5 to 3 r.p.m.
Although much past practice has involved supplying at least the majority of the heat by firing with oil or natural gas burners into the lower end of the kiln, considerable success is possible using specially controlled procedure of a recent invention whereby all or most of the heat, after initial start-up, is provided by burning, inside the kiln, the combustibles constituted in the volatile material released in the operation. In earlier procedures, some heat was usually obtained by burning the released volatile substances, and in some instances provision was made for introducing air at places along the kiln to facilitate such burning. However, in accordance with the recent invention just mentioned, it has been found eminently feasible to derive most of the heat, indeed usually all of the heat, by burning the released combustibles with a supply of air forcibly introduced into one or more regions along a central zone of the kiln.
In the process just mentioned, effort is made, by determining the temperature of the discharging coke, and the place where the principal calcining operation is occurring (the coke bed being there in an expanded state), and also by determining from time to time the actual nature or quality of the calcined coke, to make adjustments as necessary to keep the calcining temperature at a selected, desirably high value, and to locate its maximum value at a desirable place lengthwise of the kiln, whereby the traveling coke takes a certain time to reach the discharge end from such maximum temperature locality, while it somewhat decreases in temperature. This mode of control has been found to achieve a stable and very useful operation, preferably requiring no other source of heat, and even leaving a considerable amount of unburned volatile material in the gases discharged at the coke feed end of the kiln. Such unburned gaseous material can be subjected to combustion elsewhere, for utilization of its energy.
It will be understood that in all operations with a rotary kiln, the coke travels down the sloping kiln while the gases, including the products of combustion of volatiles, and unburned gases, i.e. all supplied and developed gaseous materials (whether derived from air or volatiles), are exhausted through the coke feed end of the kiln, advantageously being withdrawn under some draft, such as may be developed at a locality where the remaining combustible material is burned.
As explained above, the desired result involves removing from the charge of green petroleum coke all moisture and nearly all volatile matter while at the same time (at least in part as a separate result of heating) altering the physical nature of the coke. More specifically, the desired physical change in the coke includes removal of moisture, as stated, and change in physical structure that may be measured as the increase of real density e.g. up to about 2.10 g/cc (grams per cubic centimeter) or likewise the improvement in average crystallite size up to about 35 Angstroms, it being understood that the mean crystallite thickness of green petroleum coke may be less than 18 Angstroms.
The invention mentioned above has provided substantial improvement in calcining of petroleum coke, with economy and stability, e.g. in delivering a high throughput of reasonably uniform product of good quality while avoiding appreciable combustion of the desired carbon content. As described below, the present invention affords even more accurate and efficient control, with simple primary reliance on measurements such as the discharging coke temperature (at the downstream end of the kiln), and the feed end temperature, being that of the gas there leaving the kiln.
These further improvements, involving further principles as well (preferably) as those developed in the previous invention, and indeed capable of practice (if desired) while utilizing the procedure of such invention, are designed to achieve greater accuracy and efficiency and to minimize departures from desired kiln conditions. Indeed, a special object is to insure that the desired target values of operating conditions, i.e. both those which are measurable and those which occur inside the kiln, are in effect known, or updated as may seem necessary from time to time.
More general objects of the invention are to provide fidelity and simplicity of control while attaining essentially zero fuel cost, increase of throughput, high uniformity of production and product, and unusual stability of operation.